As is well known, the bipolar-type monolithic ICs include silicon substrates of the P-type since NPN transistors are employed as main elements in the circuit. Therefore, where the NPN transistor is operated at a saturated state, the P-type silicon substrate, N-type epitaxial layer formed on the P-type silicon substrate and P-type base region formed on the N-type epitaxial layer start to work as a collector, base and emitter, respectively, which form a parasitic PNP transistor. Further, if a pellet consisting of P-type silicon substrate is bonded to a support such as stem, tub or lead frame by an eutectic alloy of the silicon and gold, an N-type inverted layer is formed between the P-type silicon substrate and the support. As a result, the N-type inverted layer works as an emitter, P-type silicon substrate as a base, and N-type epitaxial layer as a collector so that a parasitic NPN transistor is formed. In this way, a thyristor of the PNPN structure is adversely formed by these PNP and NPN transistors.